Shock Effects in Eh6 Chondrites and Aubrites: Implications for Collisional Heating of Asteroids

EH6 Chondrites: The two principal groups of en-statite chondrites differ in their proportions of petro-logic types. Among EL chondrites, ~20% are type 3 and ~60% are type 6, whereas among EH chondrites, ~75% are type 3 and ~3% are type 6. As of this writing , there are five EH6 chondrites listed in the Meteor-itical Bulletin Database (MBD): A-882039, Y-793225 (EH6-an), Y-8404, Y-980211 and Y-980223. Because many EL6 chondrites [1] and several EH chondrites have been described as impact-melt rocks or impact-melt breccias [2-5], it is worthwhile to determine whether EH6 chondrites have also been impact melted. Most EH6 chondrites have not been extensively studied previously; I examined all of them except A-882039. Lin and Kimura [4] classified EH6 Y-8404 as an impact-melt rock because it contains euhedral enstatite grains enclosed within globules of kamacite and troil-ite. This texture was produced by crystallization from a melt and is similar to that exhibited by the Abee EH impact-melt breccia [2,6]. The reaction principally responsible for Abee's unusual mineral chemistry [7] governs the sulfidation of enstatite and the production of keilite [(Fe,Mg)S], sil-ica and metallic Fe at the expense of niningerite [(Mg,Fe)S] and troilite [FeS]: MgSiO 3 + (Mg,Fe)S + 2FeS = 3(Fe,Mg)S + SiO 2 + Fe + ½O 2. This reaction (coupled with element partitioning at low fO 2) accounts for several additional characteristics of EH impact-melt breccias: low MnO in enstatite (<0.04 wt.%), high Mn in troilite (>0.20 wt.%), the occurrence of keilite instead of niningerite, low Mn in keilite (≤4.3 wt.%), and abundant silica (>4 wt.%) [7,8]. Each of the four EH6 chondrites that I studied exhibits some of these features and thus appears to have been impact melted: Y-8404 and Y-980223 contain rare relict radial pyrox-ene chondrules and are impact-melt breccias; Y-793225 and Y-980211 are chondrule-free impact-melt rocks. All four meteorites contain euhedral enstatite grains surrounded by metal±sulfide, all four contain enstatite with <0.04 wt.% MnO, all but Y-793225 average ≥0.24 wt.% Mn in troilite, three (Y-8404, Y-980211 and Y-980223) contain keilite (with low Mn: 3.8, 3.8 and 4.1 wt.%, respectively) [8, this study], and Y-8404 (and its paired specimens) average ~12 wt.% silica [4] (whereas most EH3-5 chondrites contain ≤1 wt.% silica and many have <0.1 wt.% [7]). These data allow one of two possibilities: (1) The EH6 chondrites have been properly classified, but EH6 chondrites are impact-melt rocks and impact-melt breccias. In this case, the …